Generally, a magnetic recording medium such as a magnetic tape is prepared by coating on a support a magnetic paint comprising a magnetic powder, a binder resin, followed by drying. In conventional magnetic recording media, the magnetic layer consists of only one layer, therefore a broad frequency band region from a lower region to a higher one must be covered by one kind of magnetic powder. Particularly in a recent trend toward high density recording, a magnetic powder having a high Hc and a high BET value is used to meet requirements for a higher recording performance and lower noise in a high frequency region. Further, in a single-layered recording medium, high frequency characteristics are regarded as important, and chroma and audio output have not been paid much attention.
However, attaching too much importance to the high frequency characteristics inevitably leads to the use of a magnetic powder having a high Hc and a high BET value and brings about poor low frequency characteristics, because the magnetic recording medium comprises only one kind of magnetic powder contained in one magnetic layer.
In the magnetic recording medium for video application, there are proposed recording media having multiple magnetic layers in order to increase the recording capacity and improve magnetic recording characteristics in both of the high and low frequency regions for balancing properties. These proposals are found, for example, in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 98803/1973, 172142/1984, 64901/1976, 56228/1983, 200425/1983, 146211/1988, 300425/1988, and Japanese Patent Examined Publication Nos. 2218/1957 and 12937/1981.
In these conventional techniques, functions are separately allotted to the upper and lower magnetic layers; the upper layer is designed to cover video output, and the lower layer to cover chroma and audio output.
Japanese Patent O.P.I. Publication 200425/1983 discloses a method for improving running durability and electro-magnetic transferring properties of such magnetic recording medium having two or more magnetic layers. The magnetic recording medium has two magnetic layers, upper and lower layers. Carbon black is added only in the upper layer to raise the running durability. No carbon black is added to the lower layer for increasing the packing density of magnetic particles in the layer so that the electro-magnetic conversion properties are improved. Besides, in the case of a video tape to be used in a VHS video-deck, the recording medium is required to contain a certain amount of carbon black to make an optical transmission satisfying the predetermined value. Accordingly, the carbon black content in the upper layer is excessively high when the carbon black is added only in the upper layer in accordance with the above-mentioned technique. As a result, poor dispersibility, or aggregation in heavy cases, of magnetic powder occurs in the upper layer, which results in deteriorated S/N ratio, rough surfaces, magnetic head clogging and poor durability. Incorporation of carbon black only in the lower layer also falls in the same results.
On the other hand, a proposal is made in Japanese Patent O.P.I. Publication No. 300425/1988 to incorporate carbon black having a particle size of 80 m.mu. or more in the upper layer and one having a particle size of 40 m.mu. or less in the lower layer, in order to improve the durability without deteriorating electromagnetic conversion properties.